Transparency of spiral galaxies:

A definitive answer to a long standing question

Dust is but a small fraction of the mass of a galaxy, yet it plays a crucial role in
galactic evolution. It is on the surface of dust grains that molecules form. This gives rise to the giant molecular clouds which are a
prominent component of the interstellar medium in spiral galaxies. Dust plays a crucial role in the collapse and cooling of clouds and the
subsequent formation of stars. Thus, the study of how much dust there is in a galaxy and how it is distributed, forms a key part of any
detailed study of galactic evolution, since the star-forming process is so fundamental.

For
many years, the effects of dust on our understanding of the global properties of spiral
galaxies have been a matter of debate in the astronomical community. Using optical
observations taken from Skinakas Observatory and analyzing them with a realistic model of
spiral galaxies, we were able to give definite answers to these long standing questions.
Spiral galaxies seen from the edge are especially usefull in this respect because dust effects
are highly extragerated. As an example of this work, we present the modeling that was
done to NGC 891, which is a typical spiral galaxy, very similar to our own galaxy, viewed
edge-on.

Our
analysis has shown that typical spiral galaxies, like our own Galaxy, would be transparent if
viewed face-on, meaning that dust has little effect on our understanding of the global
properties of galaxies.In the figure to the
left, the computed distribution of the optical depth is shown for a typical spiral
galaxy observed at different inclination angles. For optical depths less than 1, the
corresponding part of the galaxy can be considered as transparent.

It
is also shown that the total amount of dust in galaxies is about ten times more than
previously thought. This has now been confirmed by recent observations in the infrared by the
ISO satellite and at submillimeter wavelengths by the SCUBA bolometer. In the figure to the
left are presented: The values of the gas mass ( top graph), the values of the dust mass
(middle graph) and their ratio (bottom graph) for the sample of observed galaxies. All masses
are units of Solar masses

Finally,
another important result is that the dust properties are similar to those in our own Galaxy,
which indicates a universal dust behaviour.